Introduction

Some Christians do not believe that Jesus gave us the substance of His body and blood in the Eucharist. These critics erroneously claim that the bread and wine we receive are merely symbols. That the substance of the Eucharist is the body, blood, soul and divinity of our Risen Lord under the appearance of bread and wine is a truth of the faith given by Christ Himself. This is one of the most profound teachings of our faith. The Holy Eucharist is an extension of the Incarnation because it is God made substantially present at each and every Mass. It is the Miracle of the Mass otherwise described as transubstantiation or the Real Presence.

Catechism
The mode of Christ's presence under the Eucharistic species is unique. It raises the Eucharist above all the sacraments as "the perfection of the spiritual life and the end to which all the sacraments tend.” In the most blessed sacrament of the Eucharist "the body and blood, together with the soul and divinity, of our Lord Jesus Christ and, therefore, the whole Christ is truly, really, and substantially contained. This presence is called 'real' - by which is not intended to exclude the other types of presence as if they could not be 'real' too, but because it is presence in the fullest sense: that is to say, it is a substantial presence by which Christ, God and man, makes himself wholly and entirely present."
- Catechism of the Catholic Church, 2nd ed., (1997), n.1374
Scripture

Ex. 12:8,11 - The paschal lamb had to be eaten to spare the first-born sons. Jesus is the true paschal Lamb.

Lev. 19:22 – In the old covenant priests made atonement for sin with the guilt offering of an animal which was consumed. What was consumed was the sacrifice, not a symbol of a sacrifice.

Zech. 9:15-16 – The sons of Zion, shall drink blood like wine and be saved. This prefigures the Eucharistic sacrifice.

Ezek. 2:8-10; 3:1-3 - God orders Ezekiel to open his mouth and eat the scroll which is the Word of God.

2 Chron. 30:15-17; 2 Chron. 35:1,6,11,13; Ezra 6:20-21 - The lamb was killed, roasted and eaten to atone for sin and restore communion with God. Jesus was stripped, pierced, and sacrificed to atone for our sin and restore communion.

Mt. 26:2; Mk. 14:12; Lk. 22:7 - Jesus' passion is connected with the Passover sacrifice where lambs were slain and eaten.

Jn. 6:35,41,48,51 - Jesus says four times that He is the bread of heaven.

Jn. 6:51-59 - Then Jesus says that the bread He is referring to is His flesh. The shocked Jews understood Jesus literally.

Jn. 6:60-67 - Jews leave because of this, and Jesus does not stop them. Jesus confirms it is truly His body and blood.

Mt. 26:26-28; Mk. 14:22,24; Lk. 22;19-20; 1 Cor. 11:24-25 - Jesus says, “this is my body and blood”…not a symbol of it.

Lk. 22:19; 1 Cor. 11:24-25 - Jesus says "do this", that is, offer the Eucharistic sacrifice,” in remembrance of Him.

1 Cor. 11:27 - You can’t profane a symbol, neither can you worship one. Paul confirms the Real Presence.

Tradition & Fathers
"They abstain from the Eucharist and from prayer, because they confess not the Eucharist to be the flesh of our Saviour Jesus Christ, which suffered for our sins, and which the Father, of His goodness, raised up again."
- St. Ignatius of Antioch, Epistle to Smyrnaeans (A.D. 110)
"For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Saviour, having been made flesh and blood for our salvation, so likewise have we been taught that the food which is blessed by the prayer of His word, and from which our blood and flesh by transmutation are nourished, is the flesh and blood of that Jesus who was made flesh."
- St. Justin Martyr, First Apology (A.D. 110-165)
"The bread over which thanks have been given is the body of their Lord, and the cup His blood..."